The present invention relates to planar arrays having broad frequency range applications for source location, source imaging or target illumination with projected beams. Prior attempts to address planar array design where the number of array elements is restricted focus on single frequency application, don't address the issue of circular symmetry, and/or are for far-field application and thus do not comprehensively address near-field, circularly symmetric, and broad band application for source mapping or target illumination with projected beams.
Regular arrays are known in the state of the art whereby array elements are placed in a periodic arrangement such as a square, triangle, or hexagonal grid. In these arrangements, adjacent elements are required to be spaced within one-half wavelength of each other to prevent the array pattern from having multiple mainlobes in other than the steered direction, a phenomenon commonly referred to as spatial aliasing or grating lobes. This half-wavelength requirement can be cost prohibitive from the standpoint of the number of array elements required in broad frequency range applications because the lowest frequency for intended use drives the array aperture size larger (to achieve adequate array resolution), while the highest frequency drives the element spacing smaller (to avoid spatial aliasing).
Irregular arrays are known in the state of the art for providing a way to address grating lobe problems inherent in regular arrays because irregular arrays eliminate periodicities in the element locations. Random arrays are known in the state of the art as one form of irregular array. Random arrays are limited in ability to predictably control worst case sidelobes. When array element location can be controlled, an algorithm may be used to determine element placement that will guarantee irregular spacing and allow for more predictable control of worst case sidelobes. Prior art contains many examples of irregularly spaced linear arrays many of which are non-redundant, that is, no spacing between any given pair of elements is repeated. Non-redundancy provides a degree of optimality in array design with respect to controlling grating lobes.
Prior art for designing irregular planar arrays is largely ad-hoc. Only a few simple examples of non-redundant planar arrays--where there is either a relatively small number of elements or a simplistic element distribution such as around the perimeter of a circle--appear to exist in prior art. Prior art appears void of non-redundant planar array design techniques for locating an arbitrary number of elements distributed throughout the array aperture (as opposed to just around the perimeter) in a controlled manner to ensure non-redundancy and circular symmetry.
It is one object of the present invention to provide a planar array design substantially absent of grating lobes across a broad range of frequencies where the available number of elements is substantially less than that required to construct a regular (i.e., equally spaced element) array with inter-element spacing meeting the half-wavelength criteria typically required to avoid grating lobe contamination in source maps or projected beams.
Another objective of the present invention is to provide a planar array design that provides circular symmetry so that the source map resolution or projected beamwidth is not substantially array-dimension (i.e., azimuthal angle) dependent.
A further object of the invention is to provide a planar array design that makes optimal use of a fixed number of array elements in the sense that the array is non-redundant.
Still another object of the invention is to provide space density tapering flexibility in the array design to allow for trade-offs in the array design between array beamwidth and sidelobe levels.
Yet another object of the present invention is to provide a general method for distributing an arbitrary number of elements on an arbitrary diameter circular planar aperture in a manner that guarantees circular symmetry and non-redundancy in the spatial sampling space.